The present invention relates to a fuel control apparatus of an engine, and particularly to an apparatus for controlling an amount of fuel supplied to the engine correspondingly to an intake pressure.
Japanese Unexamined Patent Publication No. 59-15656 has disclosed a fuel control apparatus of an engine wherein a fuel injector is provided in an intake passageway, an intake pressure sensor for detecting the intake pressure is provided in the intake passageway on the downstream side of a throttle valve and an amount of the fuel supplied from the fuel injector is controlled on the basis of the intake pressure detected by the intake pressure sensor and an engine speed.
Conventionally, there has been known a so-called sequential turboengine comprising a supercharger. The engine has primary and secondary turbosuperchargers provided in parallel with each other in an exhaust passageway thereof and an exhaust cutoff valve provided in a special exhaust passageway for the secondary turbosupercharger. When intake flow mass is small, the exhaust cutoff valve is closed so that exhaust gas of the exhaust passageway is collectively supplied to a turbine of the primary turbosupercharger. Consequently, a supercharging pressure can be increased with good rising. When the intake flow mass is great, the exhaust cutoff valve is opened so that the exhaust gas of the exhaust passageway is collectively supplied to the turbines of the primary and secondary turbosuperchargers. Consequently, the intake flow mass can be held and a proper supercharging pressure can be obtained. Referring to the above-mentioned engine, an area of the exhaust passageway is changed depending on the opening or closing of the exhaust cutoff valve. Consequently, an exhaust resistance is changed.
In the meantime, there has been known an engine comprising a variable silencer as an exhaust device. The engine has first and second exhaust outlet pipes provided in parallel with each other in an exhaust passageway thereof. The second exhaust outlet pipe has a control valve. When the engine speed is low, the control valve is closed to exhaust the gas through only the first exhaust outlet pipe. Consequently, an exhaust resistance can be increased so that noises may be prevented from being generated in a car. When the engine speed is high, the control valve is opened to exhaust the gas through the first and second exhaust outlet pipes. Consequently, the exhaust pressure can be reduced. Also referring to the above-mentioned engine, the area of the exhaust passageway is changed depending on the opening or closing of the control valve. Consequently, the exhaust resistance can be changed.
In the case where the above-mentioned engine for controlling an amount of fuel supplied thereto according to an intake pressure has an exhaust gas resistance changing means such as the above-mentioned sequential turbosupercharger or variable silencer for changing an exhaust resistance according to the operating condition of the engine, an air fuel ratio is changed when the exhaust resistance is changed (i.e., the exhaust cutoff valve or control valve is opened or closed). By way of example, in the case where the engine speed is increased with acceleration, the resistance of the intake flow through the intake passageway is reduced if the exhaust cutoff valve or control valve is opened to reduce the exhaust resistance. Consequently, when an opening of a throttle valve is not changed, the intake flow mass is excessively increased. When the opening of the throttle valve is slightly reduced, the intake pressure is reduced. Consequently, the intake flow mass does not correspond to the intake pressure. Therefore, when a supply amount of the fuel is conventionally and uniformly controlled according to the intake pressure, the air fuel ratio is deviated from a desired value. Also in the case where the engine speed is decreased with deceleration, the air fuel ratio is deviated from the desired value.